Implantable devices for controlling or monitoring various body functions represent a rapidly evolving technology. It has long been recognized that application of an electrical stimulus at a selected body location can produce a reaction by the tissue at that location. Well known examples of body function stimulators include cardiac rate pacers, bladder stimulators, phrenic nerves stimulators, and carotid sinus stimulators. Many uses are foreseen for such stimulating apparatus to either directly produce a response or to detect and control erratic stimuli produced by the body. Other implanted devices can serve as substitute organs or as monitors for detecting abnormal conditions.
It is apparent that the capability of tailoring the operating parameters of a given body stimulator to the physiological needs of the body within which the stimulator is implanted would optimize the beneficial effects of the stimulator. By way of example, cardiac pacing may require recognition and control of one or more of the following parameters:
1. Pulse rate--the pulse rate is selected from a range of rates effective to sustain life and normal activities.
2. Pulse widths--a stimulating pulse duration is selected to assure tissue response to the signal; the minimum effective pulse width generally varies as a function of such factors as myocardial fibrosis, drug therapy, and electrode selection and placement.
3. Sensitivity--the desired naturally occurring signal, generally the "QRS" complex, must be discriminable from other signal waves, such as "T" waves, to obtain proper system functioning.
4. Operating Mode--the most desirable stimulating mode of the cardiac pacer depends on the physiological status of the patient as well as the patient's environment and and a suitable mode must be selected, e.g., R-wave inhibited demand mode, R-wave synchronous demand mode, atrial-ventricular sequential demand or fixed rate mode.
It will be appreciated that physiological changes can occur even after a device, such as a cardiac pacer, is implanted such that an operating parameter change may be desired or even required to maintain effective use of the implanted device. Since modern implanted devices may be powered by a battery having a life expectancy of many years, a need has arisen to affect changes in the operating parameters without invading the body. As hereinbelow shown, the prior art has used magnetic fields and high frequency radio or other carrier signals to obtain some control over implanted cardiac pacers and other body stimulators, and has suggested transferring signals by acoustic means, light transmission, and physical pressure. The information carrying capability shown in the prior art is small, permitting limited variability. As also shown, the prior art does not provide for verifying the correctness of a received signal prior to implementing that signal. Other problems in the prior art are slow signal input, the need for a plurality of outside signals to activate a system, and/or the need for active circuits imposing a battery drain.